The production of electric power from various types of alternative energy sources such as for instance wind turbines, solar power plants and wave energy plants is not continuous. The production may be dependent on environmental parameters such as for instance wind speed (for wind turbines), insulation (for solar power plant) and wave height and direction (for wave energy plants). There is very often little or no correlation between energy production and energy demand.
One known approach to solve the problem of uncorrelated electric power production and electric power demand is to temporally store energy, which has been produced but which has not been demanded, and to release the stored energy at times at which there is a high demand. In the past there have been suggested many different methods to temporarily store energy. Suggested methods are for instance (a) mechanical energy storage methods e.g. pumped hydro storage, compressed air storage and flywheels, (b) chemical energy storage methods e.g. electrochemical batteries and organic molecular storage, (c) magnetic energy storage, and (d) thermal energy storage.
WO 9214054 A1 discloses a wind-powered energy production and storing system comprising a wind rotor in driving engagement with a power generator via transmission means, to which is also connected a heat pump for operation of at least one heat exchanger unit. The wind rotor is designed as a wind wheel having a rim in direct driving engagement with a main shaft positioned in a subjacent engine housing to which main shaft, in addition to the power generator and the heat pump, a dual circulation pump is coupled for conveying heated and cooled liquid, from a heating container and a cooling container, respectively, positioned in the engine housing to separate heat and cold storing stations. Via a steam separator and a pumping device, a steam generator may be connected to the heat storing station, which via a steam turbine drives an additional power generator for power production during periods of slack winds. The storing stations may be buried in soil having a filling of sand, stones or similar materials. One disadvantage of this wind-powered energy production and storing system is that there is a direct mechanical connection between the energy storing system and the wind turbine and that all the energy storage equipment, besides the storing stations, is placed in the wind turbine resulting in a complex mechanical arrangement of the system components. This causes the known system to be not flexible.
There may be a need for improving the temporal storage of thermal energy in particular with respect to the flexibility and the efficiency of a thermal energy storage system.